This invention relates to infrared generators of the type in which a gas is heated by mechanical compression to a temperature sufficient for radiation.
Gas molecules can be excited at sufficiently high temperature and pressure so that energy of the molecules moves from the normal energy state to a state of higher energy. As the temperature increases, there is more collision between molecules and more molecules are raised to an energy state higher than ground state, thereby absorbing energy. The molecules return rather quickly to their lower energy state, and, in doing so, spontaneously emit electromagnetic radiation. The choice of gas will determine the fundamental vibrational wavelength band. The molecular species which are most applicable for use as infrared radiators in the 2 to 6 micron spectral region are listed in Table 1.
TABLE 1 ______________________________________ Molecule Center Wavelength, microns ______________________________________ HF 2.42 HCl 3.34 CO 4.60 CN 4.83 NO 5.25 CO 4.35 N.sub.2 O.sup.2 4.50 ______________________________________
The fundamental concept for using molecular gases to provide efficient infrared radiation is that a gas layer is heated to a pressure and temperature sufficient to provide the optimum gas radiation in the fundamental molecular vibration band without exciting appreciably higher vibrational-rotational modes. This excitation of the gas molecules can be achieved by compressing the gas in a quasi-adiabatic manner (with substantially no change in heat energy) and with a reasonantly driven gas compressor mechanism whereby at least some of the unradiated internal energy in the molecular gas, as well as some of the kinetic energy of the compressor mechanism during oscillation, can be recovered for further use during succeeding radiation pulses.
As shown in my U.S. Pat. No. 3,751,666, the compressor mechanism may be of the electromagnetic type including an armature connected to a diaphragm which forms a portion of the gas chamber and is driven by an armature current coil energized from a suitable current source for supplying the necessary magnetic field. Such a compressor mechanism has a much greater efficiency in infrared spectral bands than the usual arc discharge because a relatively small amount of radiation is lost at wavelengths outside the rotational-vibrational spectral band.
Ideally, a near adiabatic system is desirable; however, there are some thermal losses during compression owing to infrared radiation, and conduction losses from the compressed gas to the walls of the gas chamber.
The total internal energy of the gas at the peak of compression is equal to the product of the known mass of the molecular gas, the specific heat of the gas at constant volume (per unit mass) and the peak temperature which is related to the initial temperature by the compression ratio to the -1 power where is the specific heat ratio or the ratio of the specific heat at constant volume to the specific heat at constant pressure. When the gas compression member is released from its position at peak compression, the member will oscillate about the equilibrium position at a natural frequency determined by the mass of the number and the compression chamber design. The system energy during compression or expansion remains substantially constant and is equal to the sum of the gas energy and the kinetic energy of the compression member.